1. Field of the Invention
The present invention relates to a semiconductor device, more specifically to a wiring board in which an electric wiring layer and an optical wiring layer are stacked on a board, and to a semiconductor device using the wiring board.
2. Description of the Related Art
As long-distance and high-capacity optical fiber transmission systems have been rapidly widespread, optical transmission technology with a capacity ranging from gigabits to terabits is being researched and developed at present. Particularly, in an optical subscriber system in the Fiber to the Home (FTTH), research for reducing manufacturing cost of an optical device module, and the like, is attempted in order to generalize the module. Specifically, there has been proposed a method of forming a V-shaped groove on a silicon substrate to facilitate the alignment of a semiconductor device with an optical fiber, and a coupling technology of the semiconductor device and the optical fiber by using a passive alignment method.
Also with regard to an LSI, much research has been conducted for enhancing performance thereof, and the operating speed and integration scale of the LSI tend to be significantly improved. It is known that a challenge on improving the performance of the LSI is the enhancement of transfer rate and packaging density in signal wiring. In other words, even if the performance of a functional device such as a transistor is enhanced, it is difficult to enhance the performance of the module unless the enhancement of the signal transfer rate and packaging density in the signal wiring is achieved. However, a delay in signal transmission is inherent in the electric signal wiring, causing a hindrance to the enhancement of the module performance. Furthermore, when the signal transfer rate and the packaging density of the signal wiring are enhanced, influence of the electromagnetic interference (EMI) significantly appears, and accordingly, it is necessary to take sufficient measures against the EMI.
As for solving such a problem regarding the electric signal wiring, optical interconnection technology is regarded as prospective. It is thought that this optical interconnection technology is applicable to many purposes such as interconnecting electronic instruments mutually, boards in the electronic instruments mutually, and chips in the boards mutually. For example, there is proposed a technology utilizing a plastic optical fiber which has a large core diameter and is easy to connect as an optical interconnection technology between the electronic instruments, a technology utilizing a flexible optical waveguide as an optical interconnection technology in the electronic instrument, or a technology utilizing the optical wave guide and optical wiring as an optical interconnection technology between the chips in the board.
In the optical interconnection technology which is effective as a next-generation high-density and high-speed communication technology, enhancement of packaging density and improvement of connection reliability in a wiring board including both of an electric wiring layer and an optical wiring layer become important. However, it has been common that, in an earlier electro-optical wiring board, the optical waveguide is formed on a multilayer wiring layer deposited on a circuit wiring board or between the multilayer wiring layers. For this reason, though no problem occurs when the thickness of the circuit wiring board serving as a support member of the optical waveguide is relatively thick, the board is deformed due to a difference in thermal expansion coefficient between the material of the optical wave guide and the material of the circuit wiring board when it is necessary to reduce the thickness of the circuit wiring board as in the case where the board is used for a mobile electronic instrument. As a result of this, a warp occurs in the optical waveguide formed on the circuit board, which causes difficulty in optically aligning the optical waveguide with an optical semiconductor module to be mounted on the circuit board. Moreover, when the optical waveguide and the multilayer wiring board, which are made of materials different in coefficient of thermal expansion, are expanded due to heat generated in the optical semiconductor module, the problem of the destruction of the board occurs. The destruction problem of the board affects a connection part of the circuit wiring board and a semiconductor module having the semiconductor device mounted thereon.
Meanwhile, with regard to an earlier semiconductor device mounting an optical semiconductor module on the wiring board, a semiconductor device, in which the optical semiconductor module is mounted on the upper most stage of the multilayer wiring layer deposited on the circuit wiring board is common. However, when the number of stacked layers of the wiring is increased, the uppermost stage of the wiring layer is deformed to a great extent, causing irregularities on the surface of the wiring layer. Accordingly, in some cases, the optical semiconductor module and the wiring board cannot be sufficiently coupled to each other. In the case of using the wiring board in which the optical waveguide is arranged between the multilayer wirings, it is necessary to arrange the optical waveguide in a portion on which a semiconductor device transferring an electric signal is not mounted. Accordingly, limitations are inherent in enhancement of a wiring density of the entire electro-optical wiring board because of layout rate determination of the optical wiring layer, which is difficult to miniaturize compared with electric wiring.